I have bought a Steca Solsum 10.10F and I am having some problems with it.
I would like to ask you guys and gals some advice to figure out if it's a problem from my side or if it a problem with the charger control.
I have 2x40Watts solar panels connected to the charger control and from there to a AGM battery.
In between this points I have devices to measure (Watts Up? Meters and other digital multimeters) Volts/Amps…. in this way I am trying to see what is going in and what is going out….
If I do not plug the solar panels to the charge controller I am reading from 19V to 20V… and after connecting it to the charge controller I read around 12.9Volts in the "input" as well on the "output"?
My AGM battery should need 14.4 to 14.7Volts in order to cycle…..
I read the user manual for this charger control and according to the specifications the DC output should go to 14.4V (Boost charge voltage)…. unfortunately, no matter the sun/sky conditions I never saw higher than 12.9 volts…
Am I doing something wrong? or what is the problem?
Best Answer
Based on the additional information in the comments and the not very helpful information from the datasheet and user manual, I'd say your system is working correctly.
Your panels cannot provide the needed power to charge the battery at 14.4V. Solar panels have a very pronounced dependency between output voltage and output current.
This shows, for a single solar cell, the dependency between output current and output voltage. \$V_{OC}\$ is the open circuit voltage which you measured to be about 19 to 20V. \$I_{SC}\$ is the short circuit current, which you haven't actually measured, but you are quite close to it I guess.
A battery has a very low resistance (often in the order of some milliohm) and as such it will draw massive amounts of charging currents. The voltage you can measure at the battery terminals is the chemical voltage plus the voltage generated over its internal resistance by the charging current.
So your solar panel will provide as much current as it can into the battery and the voltage will drop to the battery voltage + some small amount. As the battery gets charged its chemical voltage will rise and you will move to the right in that plot.
More sophisticated chargers will regulate the output current and voltage in such way, that the depicted maximum power point of the solar panel is used (\$P_{mp} @ V_{mp}, I_{mp}\$). But from the description of the Steca Solsum 10.10F it seems it's just protecting against over-charging and discharging.